1. Field of the Invention
The invention disclosed and claimed in this application pertains to a method for (1) determining the presence and location of knots and voids in wood and/or (2) for estimating wood strength. Throughout this disclosure reference will be made to wood scanning. The term wood means any solid wood product containing knots and/or voids such as lumber, veneer, etc. Specifically, the invention employs opposed multiple flat-electrode capacitors to create an electrical field through which the wood is passed. The dielectric response (capacitance and resistance) properties of knots, voids, and clear wood differ. When measured across the wood thickness the dielectric response of knots is higher and the dielectric response of voids is lower than for clear wood. Analog voltage values indicate the dielectric response properties of the wood thickness between opposing capacitor electrodes. This analog information is input to a data acquisition device and computer hardware converts the analog signals to digital values. The digital values are stored in a two-dimensional array that describes the wood dielectric response. The array is analyzed to determine which values indicate the presence of knots and which values indicate the presence of voids in the piece of wood that passed through the electric field. The computer locates and delineates the detected array values corresponding to knots and the detected values corresponding to voids with rectangles. This information may be used to evaluate wood quality, to control saws for lumber cutup, veneer sorting, or to control other special treatment apparatus and operations, etc. The invention may be used alone if only knots and voids are to be detected. On the other hand, the invention may be used with other defect detection devices such as CCD cameras, laser detectors, etc. as a component of a larger system designed to detect defect types in addition to knots and voids.
2. Background of the Prior Art
(1) Knot and Void Detection
A defect detection device is disclosed in Pullan, U.S. Pat. No. 4,922,181. The Pullan patent discloses a device for detecting defects in blister packs and laminated boards. The device disclosed by Pullan employs a single signal generating capacitor electrode and multiple receiving electrodes. By contrast, the detector of the present invention has signal generating capacitor plates of the same size as its signal receiving capacitor plates. In addition, the device disclosed by Pullan employs an operator as a component of the system to detect defects. The operator visually monitors a video screen on which the image of the dielectric constant information is displayed. By contrast, the present invention employs a computer system with specialized software to detect differences in the dielectric field that indicate the presence of knots and voids.
The device disclosed in the Pullan patent is also suited only for the detection of defects in materials for which the expected dielectric is known. Wood is a heterogeneous material with potentially large differences in moisture content and density between respective pieces such as boards, sheets of veneer, etc. Moisture content and density strongly influence the dielectric properties of individual pieces of wood.
Because the dielectric response of wood differs considerably between pieces due to density and moisture content differences, it is best, for the purpose of knot and void detection, not to assume an expected dielectric response for either clear wood or defects. The recognition of knots and voids by dielectrics in wood is best accomplished by accounting for the large between-piece differences in dielectric response. A device capable of sensing knots and voids in wood should possess a means for between-board recalibration so that the clear wood, knot wood and voids contained in each piece of wood can be recognized as such. The device disclosed in the Pullan patent lacks this recalibration function. The computer software incorporated in the system of the present invention provides the required recalibration capability for each piece of wood regardless of the degree of differences in moisture content and density between boards, sheets of veneer, etc.
Bechtel et al., U.S. Pat. No. 4,972,154, discloses a slope-of-grain detector capable of detecting knots and voids in wood. This device applies an electrical field to one surface of the wood and differences in the dielectric response allow determination of the grain angle. The electric field encounters less resistance along the grain compared to the across-the-grain travel. To determine slope-of-grain, the electrical field is sampled by a sensor composed of receiving electrodes arranged in a circular configuration around a sending electrode. By sampling at eight angles around a central receiving capacitor the actual slope-of-grain is determined based on the angle from which the highest dielectric response value is received.
The slope-of-grain device described in the Bechtel et al. patent was itself an improvement of a device disclosed in Norton et al., U.S. Pat No. 3,805,156, which corresponds to Canadian Patent No. 943,187. The device disclosed in the Norton et al. patent measures slope-of-grain using a pair of electrodes comprising a capacitor that mechanically rotates in a circular direction. As with the device disclosed in the Bechtel et al. patent, the circular design determines slope-of-grain based on the highest dielectric response sampled on the 360 degrees arc of travel.
The circular design of the devices of both the Bechtel et al. and Norton et al. patents accomplish the recalibration process required for a heterogeneous material, but in an entirely different way than does the present invention. The circular sampling of the dielectric response by the devices disclosed in the Bechtel et al. and the Norton et al. patents is done at high speed (8 kHz for the Bechtel et al. and 1200 rpm for the Norton et al.). This high speed and the fact that it is the computed difference in the dielectric response for each capacitor revolution that is used to determine grain angle results in what is essentially a recalibration of the sensor for each revolution. It matters little whether moisture content or density change even within a piece of wood if it is the highest dielectric response value per revolution that determines the wood grain angle.
Some previous non-contact devices to detect knots and voids across wood thickness have utilized microwaves (Innotec Ltd., British Patent Specification No 1,489,554; Heikkila and Osakeyhtio, Finnish Patent Publication No. 53,365; Prine, U.S. Pat. No. 3,549,986; Kinanen and Drucker, U.S. Pat. No. 4,123,702; Heikkila and Osakeyhtio, U.S. Pat. No. 4,500,835). Another device (Flatman et al., International Patent WO90/11488) employs x-rays to determine the across-wood-thickness density for detection of knots and voids. The present invention differs from those devices in the use of capacitor plates to generate an electrical field in the radio frequency range rather than in the microwave or x-ray spectrum. The advantage of the radio frequency range signal is that the flat-electrode capacitors required for signal generation and receiving are very low in cost. Equipment for generation of microwaves requires antennae that are much more costly than capacitors. Likewise, generation of radiation beams for through-wood knot and void detection requires a device with relatively high capital cost.
(2) Estimation of Wood Strength
A method of estimating wood strength is disclosed in Shajer, U.S. Pat. No. 4,941,357. That patent discloses a process comprising passing a beam(s) from a radiation source through the lumber thickness to determine the wood density profile across the lumber width. Multiple density profiles across the lumber width may be obtained along the lumber length depending on the sampling frequency and the lumber length, as the piece of lumber travels through the beam(s). Differential absorption of the radiation identifies both knots and the density of clear wood between and around the knots. Comparison of sensed clear wood density to a known base density allows estimation of the strength of the clear wood portion. The known strength of the base density and the correlated strength increase of wood as the radiation beam is attenuated allows the estimation of the strength of the sensed wood density. Sensed knot density allows differentiation of knots. Knot size and location are employed to predict the structural influence on wood strength. The combined information on strength of clear wood and knot influence on this strength allows an estimation of wood strength.
The present invention can also be used to determine wood strength by employing the electric field generated between the electrodes comprising the capacitor.
In this configuration, knot defects can be detected with the array technique previously described. However, comparison of the dielectric values of the clear wood to a base value for the purpose of estimating the density and thereby inferring the clear wood strength cannot be achieved by comparison between array values. The array dielectric response values representing clear wood must be compared to a known dielectric response base value. As previously discussed, higher moisture content values in wood of the same density will result in higher sensed dielectric response values. Therefore, for the present invention to be employed in the wood strength estimation mode an estimate of wood moisture content must be obtained. The influence of the moisture content on density must also be known to allow a correction factor to be applied to obtain an accurate estimate of wood density.
The problem with wood moisture content is not unique to the present invention. The same problem exists with the radiation method described by Shajer. Shajer, however, did not discuss this problem or a solution to this problem in his patent.
There are several non-contact in-line moisture content sensing devices available, for example, the Wagner 683 In-Line Moisture Detector. These in-line moisture content sensing devices are accurate for moisture contents between 6 and 25 percent which is an adequate range to determine moisture content in kiln-dried softwood and hardwood lumber. Veneer moisture content may be lower than 6 percent. Thus, in the case of veneer, the problem of corrections for a wide range of moisture contents does not normally exist because the range is usually between zero and 6 percent. The in-line moisture meter can be placed in-line just before or after the scanning capacitors of the present invention to determine the moisture content along each piece of wood's length. Based on the sensed moisture content, the density values of wood that were computed without regard to moisture content can be corrected.